1. Field of the Invention
This invention relates to a method and apparatus for measuring internal defects, particularly to a method and apparatus which can measure internal defects of a specimen by allowing a laser beam to be incident into a specimen from its surface and then observing the scattered light of the laser beam from inside the specimen from a different direction to the optical axis of incidence of the laser beam on the surface side of the specimen.
2. Description of the Prior Art
Method and apparatus for measuring internal defects by irradiating a laser beam or the like into an object to be inspected, acquiring its scattered light to obtain data on internal defects of the object and visualizing the defects has been known.
FIG. 16 is a sectional view showing a schematic structure of an internal defect measuring apparatus of this kind. The apparatus shown in the drawing irradiates a laser beam into the object to be inspected, measures the scattered light from a direction substantially orthogonal to the laser irradiating direction and hence visualizes the defects inside the object. This equipment has been disclosed in Japanese Patent Laid-Open No. 151243/1989.
In the apparatus shown in the drawing, a laser beam within the wavelength range of infrared rays (e.g. about 1 .mu.m) is allowed to be incident into a specimen (crystal) 191, such as a silicon wafer, from its side surface 192 as indicated by an arrow 194. The laser beam 194 is scattered by the defects 193 inside the specimen 191. The outcoming scattered light from the specimen 191 as indicated by arrows 195, is detected and magnified by a microscope 196 that is fixed at right angles to the direction of the laser beam 194. The detected and magnified image is converted to an electric signal by a TV camera 197. In this manner the internal defects of the specimen 191 are detected and visualized.
Incidentally, the defects 193 inside the object 191 are depicted on a relatively large scale for the purpose of description in FIG. 16, however, the defects to be detected by internal defect measuring apparatus of this kind are actually very small defects of approx. 20-50 .mu.m in diameter. This also holds true of the defects shown in later drawings.
In the internal defect measuring method or apparatus described above, however, the apparatus is generally in a 90.degree. scattering arrangement such that the direction of the incident laser beam is at 90.degree. to the observation direction of the scattered light as shown in FIG. 16. This is because this arrangement is the one which minimizes distortion of the image and stray light during observation.
In accordance with the conventional system for measuring the internal defects of the object by use of 90.degree. scattering, however, the laser beam is allowed to be incident from the side surface of the object. Therefore, the side surface must be smooth enough that the laser beam can be incident into the object. The side surface of a semiconductor wafer (side surface 192 in FIG. 16), for example, is not generally flat and it is not possible to introduce the laser beam to the inside from the side surface in accordance with the internal defect measuring method (apparatus) shown in FIG. 16. It is therefore necessary to cut or cleave the side surface and to polish it. There is thus the drawback that the specimen must be destroyed in some cases.
Even when defects near the center of a flat sheet-like object such as a semiconductor wafer are measured, the measurement portion must be cut and in this sense, the specimen must be partially destroyed, as well.